Method for maximizing uniform effluent flow through a waste water treatment system

ABSTRACT

A screen decanter for decanting liquid from a reservoir, comprising at least one rack comprising screens and baffles forming the sides of a cavity; a frame attached to the screens and baffles and providing a barrier so that liquid cannot pass from outside into the cavity without passing through the screens; a patterned perforated drain pipe inside the cavity and leading to an opening through which liquids may drain out from the cavity. The pattern of the openings counteracts the hydrostatic head within the rack such that flow through the screens is uniform at all depths of immersion in the liquid reservoir. Preferably, the screens have a porosity of about 50 micrometers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of a pending U.S. patentapplication Ser. No. 14/985,502 (the '502 Application), filed Dec. 31,2015, which is a Continuation-In-Part of [abandoned] U.S. patentapplication Ser. No. 14/471,247 (the '247 Application), filed Aug. 28,2014, which is a Continuation-In-Part of a pending U.S. patentapplication Ser. No. 14/142,197, filed Dec. 27, 2013 (the '197Application). All of the foregoing applications are hereby incorporatedherein by reference in their entirety for all purposes.

TECHNICAL FIELD

The present invention relates to the field of waste water treatment;more particularly, to a high-capacity waste water treatment system; andmost particularly, to apparatus and method for creating uniform effluentflows through micro-screens to maximize flow volumes and minimizedowntime.

BACKGROUND OF THE INVENTION

In developed and developing countries, primary treatment anddisinfection of waste water discharges from collection systems and wastewater treatment facilities is the first step to improving water quality.Typically, secondary and tertiary waste water treatment processes areadded to provide additional treatment of the primary effluent.

Primary treatment removes large and dense solids via screening andgravitational settling, allowing neutrally-buoyant matter to pass intothe secondary treatment process or receiving body of water. Primarytreatment utilizing gravitational settling or clarification isrecognized as removing 20-33% of the organic load as measured inBiochemical Oxygen Demand (BOD). Secondary treatment removes another50+% of the organic load by converting the BOD to biomass, in the formof bacteria, and CO₂.

Secondary treatment provides an environment of adequate temperature,volume, mixing, and oxygen, or the absence of oxygen in anaerobicprocesses, to sustain the bacterial population necessary to consume theBOD and nutrients remaining in the waste water′ after primary treatment.New organic matter enters the treatment facility continuously so aportion of the existing bacterial population is removed from the processto promote the growth of new bacteria. The effectiveness of primarytreatment directly affects secondary process or the receiving body ofwater if discharged from the collection system.

Primary clarifiers or settling basins are recognized as being the mosteconomical means to reduce BOD as there is little energy required and nobiomass to maintain. Primary treatment creates no biomass' and thereforerequires no aeration energy; no process controls to monitor the biomassto determine the health of the biomass; no separation or removal ofbacteria by moving to a side-stream digester; no aeration of thedigester; and no dewatering and disposal of surplus bacteria, alsocalled secondary sludge. The lack of complexity of primary treatment iswell suited for developing nations to promote recovery of surface watersand aquifers, resulting in a reduction in health issues.

Existing primary clarifiers may be circular or rectangular tanks and arevolumetrically and geometrically sized to provide a horizontal fluidvelocity lower than the solids settling velocity. The horizontal traveltime and distance of the liquid from the inlet to the effluent weir ordecanter must be greater than the settling time and distance of thesuspended solids so that solids settle out prior to reaching theeffluent weir or decanter. These settled solids contain a majority ofthe BOD in raw sewage. The effectivenss of this first stage is importantbecause the More solids that exit the primary clarifier, the lower theBOD entering the secondary treatment process or the effluent-receivingbody of water.

The '197 Application discloses an improved screen decanter with anultrafine screen (also referred to herein as a screen box or “SBX”) inthe form of a box, oval, or cylinder that is controllably driven in thevertical direction to optimize the exposure of the screen to varyingwastewater levels and that can be lifted from the wastewater forbackflushing and sterilization in a dedicated overhead apparatus.Because the motion of the screen assembly is only vertical, the requiredfootprint in the tank can be relatively small. An air scour headerprovides air bubbles to air scour the screen surface. The applicationfurther discloses a low profile screen box useful for wastewater systemshaving high flows, limited surface area to place a screen box, and/orshallow active tank volumes of existing primary clarifiers, wheremultiple screen boxes or racks may be ganged in parallel to provide thenecessary screen surface area at a controlled screen loading rate.

The '197 Application further discloses a deflector plate that increasesthe horizontal travel distance to the screen surface for solids that maybe disturbed and start to move towards the screen.

A baffled lifting column and combined stub effluent drain pipe for anSBX are also disclosed in the '197 Application. The baffled liftingcolumn is a slotted or perforated circular pipe that is connected to aneffluent pipe or hose below the weir or dcanter. The lifting columns arecentered in the SBX with openings to encourage flow distribution throughthe screen. A long rectangular screen rack has 3 lifting columnscentered and equally spaced in the screen racks. Preferably, the openarea of the baffled lifting column is lowest at the bottom and increaseswith elevation, creating head loss at the lower portion of the liftingcolumn to equalize travel distance and pressure, and thus to equalizeflow through the screen from the lowest point to the highest point ofliquid contact.

An apparatus and method for simply and automatically preventing foulingof the upstream surface of any screen assembly is disclosed by the '247Application.

In continued use of fine-screen apparatus in waste water treatment, itis important to address potential fouling and blockage of the screeningas a potential operational problem that can lead to inefficiency becauseof time lost to clean and/or replace clogged screens. Additionalmaintenance issues are typical in prior art operations, especially inhigh flow-volume situations such as municipal waste water treatmentplants.

What is needed in the art is a method for maximizing decanter throughputby increasing flow uniformity and hence total flow through the screenelement and that increases operational efficiency by increasing the timeinterval between required screen cleanings and/or screen replacements.

It is a principal object of the invention to maximize flow rate andvolume of waste water effluent through a wastewater treatment systemwithout fouling decanter screens prematurely or increasing the overallfootprint of a screen box assembly, and thus without increasing theoverall footprint of the primary treatment facility.

To enable this principle, it is a further object of the invention tocontrol the waste water flow through a screen decanter so that allportions of all screens experience approximately the same flow rate,thus minimizing localized, high-peak flow regions that can clog portionsof fine screens, and maximizing decanter throughput.

SUMMARY OF THE INVENTION

The present invention is directed to a method wherein an improved screendecanter in, a waste water treatment system is provided with screens anddrain standpipes configured to maximize total flow rate through thescreens by equalizing flow rates through unit areas of the screens atall immersed levels.

In accordance with one embodiment of the present invention, a screendecanter comprises a rack including a frame; a plurality of screensattached to the frame and positioned to define opposite sides of acavity; baffles attached to the frame and positioned to define twoadditional opposite sides of the cavity; and at least one perforateddrain standpipe disposed within the cavity. Preferably, the screens havea porosity between 25 micrometers and 75 micrometers, most preferablyabout 50 micrometers. The screen decanter provides a barrier to solidslarger than the employed porosity so that liquid passes from thesettling tank into the cavity exclusively through the screens. Filteredliquid is drained from the cavity through a pattern of openings alongthe length of the drain standpipe. The pattern is configured tocounteract the range of hydraulic head within the cavity to provideessentially equal flow through the screen and drain standpipes at alldepths of immersion. End baffles, pipe positions, and angles that thepipe perforations face, work together to further even out horizontalflow patterns that complement the aforementioned uniform flow at variousdepths within the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a first isometric view from above of an embodiment of a screendecanter (SBX) in accordance with the present application;

FIG. 2 is a second isometric view in partial cutaway of the screendecanter shown in FIG. 1;

FIGS. 3-9 are elevational views of alternate configurations ofperforations in a drain standpipe; and

FIG. 10 is a schematic horizontal cross-sectional view of a portion of ascreen decanter showing a currently preferred orientation of a drainstandpipe having a pattern of perforations with respect to the directionof effluent flow through a decanter screen.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following description, specific elements are set forth inorder to provide a more thorough understanding of the invention.However, in some embodiments the invention may be practiced without someof these elements. In other instances, well known elements have not beenshown or described in detail to avoid unnecessarily obscuring thedisclosure. Accordingly, the specification and drawings are to beregarded as illustrative rather than restrictive. It is to be furthernoted that the drawings may not be to scale.

Referring now to FIGS. 1 and 2, a preferred embodiment 01 of a screendecanter in accordance with the present application comprises at leastone rack 10, with the embodiment shown comprising three racks 10. Eachrack 10 comprises first and second screens 12 forming opposite sides ofa cavity 14 within which one or more drain standpipes 16 are disposed todraw waste water through screens 12. A pattern of openings along thelength of each of the drain standpipes 16 is formed to counteract therange of hydraulic head within the cavity to provide essentially equalflow through the screen and drain standpipe at all depths of immersionof the screen. Exemplary patterns of openings are shown in FIGS. 3-9 andare described in greater detail below.

Screens 12 are preferably mounted as coplanar pairs, two to a side, andrepeated on opposite sides of each cavity. The screens on opposite sidesof cavity 14 are parallel and in a currently preferred embodiment areeight inches apart, although size and spacing of screens 12 may varydepending on the application. To promote uniform flow of liquid betweenracks 10, the racks also are preferably eight inches apart. In apreferred embodiment, the screens 12 are ultrafine screens ofapproximately 50 micrometers porosity.

Baffles 18 are positioned at the ends of the cavity 14 defined by thescreens 12. In a preferred embodiment, the baffles 18 aresemi-elliptical, with the combination of screens 12 and baffles 18defining cavity 14 with an elongate base and parallel sides. This shapeof screens and baffles smoothes out the flow of waste water in thevicinity of rack 10 in a manner that minimizes turbulent flows in thevicinity of the ends. A frame 20, attached to and supporting the screens12 and baffles 18, forms the bottom and the remaining sides of the rack10 and provides a barrier so that liquid passing from outside of therack 10 into the cavity 14 passes exclusively through the screens 12during decanting.

In operation, a currently preferred vertical placement of the screendecanter 01 relative to the surface of liquid in a tank in a waste watertreatment system is such that the screen decanter is submerged only tothe depth necessary to bring liquid approximately to the top of thescreens 12 but no further, to prevent unfiltered fluid from spillingover the top of the frame 20 into cavity 14.

Referring to FIGS. 1-2, at least one drain opening 22 is located at thebottom of cavity 14 through which liquids may flow from cavity 14.

At least one drain apparatus 16, currently preferred as a cylindricalstandpipe, is located inside the cavity 14 within the rack 10, forming achannel for the flow of liquid from inside the cavity 14 to drainopening 22. In a preferred embodiment, several such drain standpipes 16comprising tubes of constant diameter are evenly spaced within the rack10 to promote uniform flow of liquid through each screen 12. In apreferred embodiment, the drain standpipes 16 are positioned on or neara line down the center of the rack 10. Fluid dynamic modeling shows thatthe combination of curved end baffles 18, parallel coplanar screens 12,and evenly spaced patterned drain standpipes 16, all being properlydimensioned for the chosen operating conditions, provides uniform flowof effluent through screens 12 across the width and height thereof.

Referring to FIGS. 3-10, various patterns of openings 24 (holes 24 a,vertical slots 24 b, tapering or of variable length horizontal slots 24c, and screening 24 d) are shown. The cross-sectional areas of theopenings 24 increase with increasing height along the length of thedrain pipe 16 to promote uniform flow top to bottom of each screen 12 bycounteracting the hydraulic head of filtered effluent within cavity 14.

Referring to FIG. 7, in a presently preferred embodiment, a plurality ofcircular openings 24 a of differing diameter are vertically spaced alongthe length of each drain standpipe 16, graduated by height to receivedecanted fluid, and increasing in diameter, with smaller diameter holes24 a at the bottom and larger diameter holes 24 a at the top, promotinguniform flow vertically across each screen 12.

Referring to FIG. 8, in an alternate embodiment, a plurality of circularopenings 24 a of approximately the same diameter are vertically spacedalong the length of each drain standpipe 16, with the cross-sectionalarea of the perforations 24 increasing vertically with increasing heightalong the length of the pipe 16 (i.e., with the number and/or diameteropenings 24 a generally increasing with increasing height along thelength of the drain pipe 16). As shown in FIG. 10, in a currentlypreferred embodiment, the centerline 28 of openings 24 a is generallyoriented at approximately ninety degrees (normal) to the direction ofeffluent flow 30 through the screens 12; that is, the openings do notface the screens 12 directly, promoting uniform flow horizontallythrough each screen 12. The diameter and orientation of the drainstandpipes 16 is such that they create minimal turbulence within therack 10 during decanting.

Referring further to FIGS. 1 and 2, in a preferred embodiment, screendecanter 01 further comprises a deflector plate 26 disposed at thebottom of racks 10 to suppress vertical motion of liquid in a tank belowthe decanter 01, in effect preventing larger previously-settled BODparticles from moving up in the tank and fouling the screens 12.

In a preferred embodiment, an air plenum 28 is attached to a lowerregion of each frame 20, each air plenum 28 being supplied from a sourceof compressed gas and also provided with exit slots 30 so that gasbubbles exiting the air plenum 28 through the exit slots 30 flow along,near, and through the surfaces of the screens 12 of the screen boxassembly to scour and clean the screens 12.

While the invention has been described by reference to various specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

What is claimed is:
 1. A method for controlling the flow rate of aliquid being decanted from a reservoir, comprising the steps of: a)partially immersing a screen decanter comprising a screen in said liquidby adjusting the elevation of said screen decanter relative to a surfacelevel of said liquid; and b) controlling the depth of immersion of saidscreen decanter in said liquid to limit an area of said screen that isimmersed in said liquid.
 2. A method in accordance with claim 1 whereinsaid screen is completely immersed in said liquid.
 3. A method fordecanting a reservoir containing a liquid comprising: a) providing ascreen decanter having a three-dimensional structure, including abottom, one or more sides, each of which comprises one or more decanterscreens, and a drain apparatus disposed in an opening in said bottom fordraining said liquid from within said screen decanter, wherein saiddrain apparatus includes a plurality of openings; b) immersing saidscreen decanter in said liquid; and c) arranging said plurality ofopenings in a pattern along a vertical length and a partialcircumference of said drain apparatus to provide substantially equalrates of flow per unit area across an immersed portion of said decanterscreens of said screened decanter.
 4. The method of claim 3 wherein saidscreen decanter is immersed such that said decanter screens of saidscreened decanter are fully submerged in said liquid.
 5. The method ofclaim 3 further comprising a method to control the flow rate of saidliquid decanted from said reservoir, the method comprising: a) partiallyimmersing said screen decanter in said liquid; and b) controlling thedepth of immersion of said screen decanter in said liquid to limit thearea of said decanter screen immersed in said liquid.
 6. The method ofclaim 5 wherein said decanter screen is completely submerged in saidliquid.